Producing method and producing apparatus of press formed product

ABSTRACT

A producing method of the press formed product includes a preparation step of preparing a metal sheet as a starting material, and a press working step of performing press working on the starting material by using a punch, and a pad and a die that are paired with the punch. The press working step includes a first step of forming, by the punch and the pad, the entire region of a top panel part, the entire region of each of a ridge line parts including the shape-changing part, and a region up to a predetermined height from the border between a vertical wall part and the ridge line part of a region of each of the vertical wall parts, and a second step of forming, subsequent to the first step, by the punch and the die, the remaining regions with the starting material sandwiched between the punch and the pad.

TECHNICAL FIELD

The present invention relates to a method and an apparatus for producinga press formed product used in automobiles, various vehicles other thanautomobiles, home appliances, vessels, architectural materials, etc.

BACKGROUND ART

For example, the car body of an automobile includes various structuralmembers (examples: floor cross members, side sills, side members, etc.).Press formed products, which use metal sheets such as steel sheets astheir starting materials, are heavily used in the structural members.Due to the advantages over material rupture, shape fixability, andproducing cost at the time of press forming, in many cases, a pressformed product is obtained by performing press working on a metal sheetthrough bending forming. The press formed product has, for example, achannel cross-section shape or a hat-shaped cross-section shape. Thepress formed product having a channel cross-section includes a top panelpart and a pair of vertical wall parts extending from the top panelpart. The press formed product having a hat-shaped cross-section furtherincludes a pair of flange parts extending from the respective verticalwall parts.

FIG. 1 is an illustration schematically showing a configuration exampleof a press forming apparatus that performs general bending forming. Asshown in FIG. 1, a press forming apparatus 1 is an apparatus forproducing a press formed product having a channel cross-section or ahat-shaped cross-section. FIG. 1 illustrates the case of producing apress formed product having a hat-shaped cross-section (see a dashedline in FIG. 1). The press forming apparatus 1 includes a punch 2 as alower die, and includes a die 3 and a pad 5, which are paired with thepunch 2, as an upper die. The pad 5 is supported by the die 3 or by anupper die holder or slide integrally operated with the die 3 via apressure member 4. The pad 5 can be received in the die 3, andconstitutes a part of the die 3 with being received in the die 3.

The bending forming of forming a metal sheet 6 into a press formedproduct by such press forming apparatus 1 is performed as follows.Before the forming, the metal sheet 6 is sandwiched between the punch 2and the pad 5. That is, before staring the pushing-in of the metal sheet6 into the die 3 by the punch 2, a portion of the metal sheet 6, whichportion is formed into a top panel part of the press formed product, isrestrained by the punch 2 and the pad 5. In this state, the die 3 isdescended to a bottom dead point. In this manner, the top panel part ofthe press formed product is formed along a top surface (a front endsurface) 2 a of the punch 2. Vertical wall parts are formed along sidesurfaces 2 b of the punch 2. Ridge line parts are formed between the toppanel part and the vertical wall parts. The ridge line parts connectingthe top panel part to the vertical wall parts are formed along punchshoulder parts 2 c of the punch 2. Hereinafter, such bending forming isreferred to as the pad bending forming.

In these years, it is required for automobiles to save the car bodyweight, so as to improve the fuel economy, which contributes toprevention of global warming. Further, the improvement of the safety atthe time of collision accident is required. Because of theserequirements, a high-strength steel sheet having a tensile strength of440 MPa or more is used as the metal sheet 6, which is the startingmaterial of the structural member. For example, a 590 MPa-classhigh-strength steel sheet is also used, and in some cases, a 980MPa-class high-strength steel sheet, or even a 1180 MPa-classhigh-strength steel sheet is used.

Additionally, the shapes of the structural members may become relativelycomplex. This is because of the design constraints, such as preventionof the interference between the structural members and the othermembers, bonding between the structural members and the other members,securing of a desired space, etc.

FIGS. 2 to 8 are perspective views showing examples of press formedproducts having relatively complex shapes. Among these figures, FIG. 2shows a first exemplary press formed product 7 formed from ahigh-strength steel sheet having a tensile strength of 590 MPa or more(an alloyed hot-dip galvanized steel sheet made of DP (Dual Phase) steelhaving a sheet thickness of 1.2 mm). FIGS. 3 to 8 show second to seventhexemplary press formed products 7 formed from a high-strength steelsheet having a tensile strength of 440 MPa or more (an alloyed hot-dipgalvanized steel sheet, a non-plated steel sheet, etc. having a sheetthickness of about 1.2 mm), respectively.

Each of the cross-section shapes of the first to seventh exemplary pressformed products 7 is a hat shape. That is, each of the press formedproducts 7 includes a top panel part 7 a, vertical wall parts 7 cextending from the left and right ends of the top panel part 7 a, ridgeline parts 7 b between the top panel part 7 a and the vertical wallparts 7 c, and flange parts 7 d extending from the lower ends of therespective vertical wall parts 7 c. Further, the press formed product 7includes shape-changing parts 9 in parts of the ridge line parts 7 b.

In the first exemplary press formed product 7 shown in FIG. 2, theshape-changing parts 9 are provided in both of the ridge line parts 7 bon both ends of the top panel part 7 a. In these shape-changing parts 9,the height of the ridge line parts 7 b is changed at local portionslocated in almost middle of the lengthwise direction of the press formedproduct 7. In this case, a step height part 8 is formed in the top panelpart 7 a in a region connecting the shape-changing parts 9 to eachother.

In the second exemplary press formed product 7 shown in FIG. 3, theshape-changing parts 9 are provided in both of the ridge line parts 7 bon both ends of the top panel part 7 a. In these shape-changing parts 9,the height of the ridge line parts 7 b is changed in a wide area almostin the middle of the lengthwise direction of the press formed product 7.In this case, the height of the top panel part 7 a gently changescorresponding to the positions of the shape-changing parts 9.

In the third exemplary press formed product 7 shown in FIG. 4, theshape-changing part 9 is provided in one of the ridge line parts 7 b onboth ends of the top panel part 7 a. This shape-changing part 9 changethe arc length in a cross section of the ridge line part 7 b at a localportion almost in the middle of the lengthwise direction of the pressformed product 7. In this case, the angle between the top panel part 7 aand the vertical wall part 7 c changes corresponding to the position ofthe shape-changing part 9.

In the fourth exemplary press formed product 7 shown in FIG. 5, theshape-changing parts 9 are provided in both of the ridge line parts 7 bon both ends of the top panel part 7 a. In these shape-changing parts 9,the ridge line parts 7 b are twisted in a wide area almost in the middleof the lengthwise direction of the press formed product 7. In this case,the top panel part 7 a and the vertical wall parts 7 c are twistedcorresponding to the positions of the shape-changing parts 9.

In the fifth exemplary press formed product 7 shown in FIG. 6, theshape-changing parts 9 are provided in both of the ridge line parts 7 bon both ends of the top panel part 7 a. In these shape-changing parts 9,the ridge line parts 7 b are warped in the width direction of the pressformed product 7 in a wide area almost in the middle of the lengthwisedirection of the press formed product 7. In this case, the top panelpart 7 a and the vertical wall parts 7 c are warped in the widthdirection of the press formed product 7 corresponding to the positionsof the shape-changing parts 9.

In the sixth exemplary press formed product 7 shown in FIG. 7, theshape-changing part 9 is provided in one of the ridge line parts 7 b onboth ends of the top panel part 7 a. In the shape-changing part 9, thisridge line part 7 b is hollowed in the width direction of the pressformed product 7 at a local portion almost in the middle of thelengthwise direction of the press formed product 7. In this case, thetop panel part 7 a and the vertical wall parts 7 c are hollowed in thewidth direction of the press formed product 7 corresponding to theposition of the shape-changing part 9.

In the seventh exemplary press formed product 7 shown in FIG. 8, theshape-changing part 9 is provided in one of the ridge line parts 7 b onboth ends of the top panel part 7 a. In this shape-changing part 9, theradius of curvature of a cross section of the ridge line part is changedin an area from almost the middle to one end of the lengthwise directionof the press formed product 7. In this case, the width of the top panelpart 7 a and the height of the vertical wall part 7 c are changedcorresponding to the position of the shape-changing part 9.

When cold producing the first exemplary press formed product 7 shown inFIG. 2 from a high-strength steel sheet by the pad bending forming shownin FIG. 1, the following problems occur. In the press formed product 7,a wrinkle is generated in the regions of the shape-changing parts 9 ofthe ridge line part 7 b and in the regions in the vicinity thereof (thevertical wall parts 7 c, etc.).

Such a wrinkle similarly occurs even in the second to seventh exemplarypress formed products 7 shown in FIGS. 3 to 8.

Aiming to suppress the occurrence of such a wrinkle reduces the degreeof freedom in designing the structural members. This is because thevarious dimensions of the shape-changing parts 9 in the ridge line parts7 b are restricted in accordance with the press formability of ahigh-strength steel sheet, which is a starting material.

The prior arts for preventing the occurrence of a wrinkle in pressformed products are disclosed in, for example, Japanese PatentApplication Publication No. 2010-115674 (Patent Literature 1) andJapanese Patent Application Publication No. 2012-024837 (PatentLiterature 2).

Patent Literature 1 discloses the technique for applying concavo-convexbeads to vertical wall parts and flange parts. With the application ofthe beads, the line length difference is reduced that is generated in anend part of a starting material before and after bending forming. Inthis manner, the generation of wrinkles is suppressed.

Patent Literature 2 is directed to the production of a press formedproduct having a hat-shaped cross-section shape and bent in the widthdirection, and discloses the technique for suppressing wrinklesgenerated in outer vertical wall parts, etc. of a bending portion of thepress formed product. In this technique, an excess thickness portion isprovided only on the outer side of the bending portion. At the time ofpress forming, the excess thickness portion is bent in the directionopposite to the press direction by upper and lower dies, and is alsosandwiched between the dies. In this manner, tensile force is applied tothe outside of the bending portion, and the generation of wrinkles issuppressed.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.2010-115674

Patent Literature 2: Japanese Patent Application Publication No.2012-024837

SUMMARY OF INVENTION Technical Problem

According to the technique disclosed in Patent Literature 1,concavo-convex shapes of beads appear on the press formed product.Therefore, the application of this technique is limited to theproduction of a special press formed product.

The technique disclosed in Patent Literature 2 cannot be necessarilyapplied to the production of all of the above-described first to seventhexemplary press formed products 7. Additionally, according to thistechnique, because the excess thickness portion exists, the yieldreduction cannot be avoided.

As described above, according to the prior art, when forming the pressformed product having a shape-changing part in a part of the ridge lineparts from a high-strength steel sheet having a tensile strength of, forexample, 440 MPa or more, it is impossible to reduce the generation ofwrinkles in the region of the shape-changing part and the region in thevicinity thereof.

An object of the present invention is to provide a producing method anda producing apparatus of a press formed product that can reduce thegeneration of wrinkles in the region of the shape-changing part and theregion in the vicinity thereof, even in the case where, for example, ahigh-strength steel sheet is used as the starting material, when formingthe press formed product having a channel cross-section or a hat-shapedcross-section and including a shape-changing part in a part of the ridgeline parts.

Solution to Problem

(1) A producing method according to an embodiment of the presentinvention is a method for producing a press formed product from astarting material.

The press formed product includes a top panel part, vertical wall partsrespectively extending from both ends of the top panel part, and ridgeline parts between the vertical wall parts and the top panel part, andincludes a shape-changing part in a part of the ridge line parts.

The producing method includes:

a preparation step of preparing a metal sheet as the starting material;and

a press working step of performing press working on the startingmaterial by using a punch, and a pad and a die that are paired with thepunch.

The press working step includes:

a first step of forming, by the punch and the pad, at least a regionadjacent to the shape-changing part of a region of the top panel part,at least a region of the shape-changing part of a region of each of theridge line parts, and at least a region adjacent to the shape-changingpart of a region of each of the vertical wall parts and up to apredetermined height from a border between the vertical wall part andthe ridge line part; and

a second step of forming, subsequent to the first step, by the punch andthe die, the remaining regions, with the starting material sandwichedbetween the punch and the pad.

In the producing method according to the above-described (1), it ispreferable that the predetermined height is 2 mm or more from the borderbetween the vertical wall part and the ridge line part, and is equal toor less than a half of the total height of the vertical wall part.

The producing method of the above-described (1) can employ the followingconfiguration.

In the first step, the entire region of the top panel part, the entireregion of each of the ridge line parts, and a region up to thepredetermined height from the border between the vertical wall part andthe ridge line part of a region of each of the vertical wall parts areformed by the punch and the pad.

In the second step, a region beyond the predetermined height of theregion of each of the vertical wall parts is formed by the punch and thedie.

The producing method of the above-described (1) can employ the followingconfiguration.

The shape-changing part of the ridge line part is at least one of (a) to(f) as follows:

(a) the height of the ridge line part is changed;

(b) the arc length in the cross section of the ridge line part ischanged;

(c) the ridge line part is twisted;

(d) the ridge line part is bent in the width direction;

(e) the ridge line part protrudes or is hollowed in the width direction;and

(f) the radius of curvature in the cross section of the ridge line partis changed.

(2) A producing apparatus according to an embodiment of the presentinvention is an apparatus for producing a press formed product from astarting material.

The press formed product includes a top panel part, vertical wall partsrespectively extending from both ends of the top panel part, and ridgeline parts between the vertical wall parts and the top panel part, andincludes a shape-changing part in a part of the ridge line parts.

The producing apparatus includes a punch, and a pad and a die that arepaired with the punch.

The punch includes a front end surface corresponding to a region of thetop panel part, side surfaces corresponding to regions of the respectivevertical wall parts, and punch shoulder parts corresponding to regionsof the respective ridge line parts.

The pad includes a bottom surface corresponding to at least a regionadjacent to the shape-changing part of a region of the top panel part, acorner part corresponding to at least a region of the shape-changingpart of a region of each of the ridge line parts, and an inner sidesurface corresponding to at least a region adjacent to theshape-changing part of a region of each of the vertical wall parts andup to a predetermined height from a border between the vertical wallpart and the ridge line part.

(3) A producing apparatus of a press formed product according to anembodiment of the present invention includes a punch, a pad and a die.

The punch includes a front end surface, side surfaces, and punchshoulder parts provided between the front end surface and the sidesurfaces. The punch shoulder parts are provided with a shape-changingpart in a part of a longitudinal direction of the punch.

The pad includes a bottom surface opposing the front end surface of thepunch, inner side surfaces opposing a part of the side surfaces of thepunch, and corner parts provided between the bottom surface and theinner side surfaces and opposing the punch shoulder parts.

The die includes inner side surfaces opposing, of regions of the sidesurfaces of the punch, regions except for regions opposing the innerside surfaces of the pad.

In the producing apparatus according to the above-described (2), it ispreferable that the predetermined height is 2 mm or more from the borderbetween the vertical wall part and the ridge line part, and is equal toor less than a half of the total height of the vertical wall part.

The producing apparatus according to the above-described (2) can employthe following configuration.

The bottom surface of the pad corresponds to the entire region of thetop panel part. Each of the corner parts of the pad corresponds to theentire region of each of the ridge line parts. Each of the inner sidesurfaces of the pad corresponds to a region up to the predeterminedheight from the border between the vertical wall part and the ridge linepart of a region of each of the vertical wall parts.

The die includes inner side surfaces corresponding to regions beyond thepredetermined height of regions of the respective vertical wall parts.

In the case of this producing apparatus, it is preferable that thefollowing configuration is employed.

The die is divided to correspond to each of the vertical wall parts.

The producing apparatus includes a die moving mechanism that moves eachof the dies toward a respective one of the side surfaces of the punchafter completion of pushing-in of the starting material into the pad bythe punch.

Advantageous Effects of Invention

According to the present invention, it is possible to reduce theoccurrence of a wrinkle in the region of the shape-changing part and theregion in the vicinity thereof, even in the case where, for example, ahigh-strength steel sheet is used as the starting material, when formingthe press formed product having a channel cross-section or a hat-shapedcross-section including the shape-changing part in a part of the ridgeline part.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an exemplarystructure of a press forming apparatus performing general bendingforming.

FIG. 2 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 3 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 4 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 5 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 6 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 7 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 8 is a perspective view showing an example of a press formedproduct having a relatively complex shape.

FIG. 9 is a perspective view showing an example of the structure of aproducing apparatus used in producing a press formed product accordingto a first embodiment of the present invention.

FIG. 10 is a perspective view showing an example of the structure of aproducing apparatus used in producing the press formed product accordingto the first embodiment of the present invention.

FIG. 11 is a perspective view showing an example of the structure of aproducing apparatus used in producing the press formed product accordingto the first embodiment of the present invention.

FIG. 12A is a diagram showing the status of a press working step by theproducing apparatus of the first embodiment, and showing the statebefore starting the forming.

FIG. 12B is a diagram showing the status of the press working step bythe producing apparatus of the first embodiment, and showing the stateof the early phase of the forming.

FIG. 12C is a diagram showing the status of the press working step bythe producing apparatus of the first embodiment, and showing the stateat the completion of the forming.

FIG. 13A is a diagram showing the status of a press working step by aproducing apparatus of a second embodiment, and showing the state beforestarting the forming.

FIG. 13B is a diagram showing the status of the press working step bythe producing apparatus of the second embodiment, and showing the stateof the early phase of the forming.

FIG. 13C is a diagram showing the status of the press working step bythe producing apparatus of the second embodiment, and showing the stateat the completion of the forming.

FIG. 14A is a diagram showing the status of a press working step by aproducing apparatus of a third embodiment, and showing the state beforestarting the forming.

FIG. 14B is a diagram showing the status of the press working step bythe producing apparatus of the third embodiment, and showing the stateof the early phase of the forming.

FIG. 14C is a diagram showing the status of the press working step bythe producing apparatus of the third embodiment, and showing the stateat the completion of the forming.

FIG. 15 is a graph showing the study result of an example.

DESCRIPTION OF EMBODIMENTS

As a result of a great deal of considerations for solving theabove-described objects, the inventors have obtained the followingfindings.

When forming, by the pad bending forming, a press formed product havinga channel cross-section or a hat-shaped cross-section including ashape-changing part in a part of a ridge line part, a region of amaterial metal sheet, which region is formed into a top panel part, isrestrained by a punch and a pad from beginning to end. On the otherhand, the region formed into the ridge line part and the region formedinto the vertical wall part are processed sequentially with thepushing-in of the punch into the die, after the region of the top panelpart is restrained by the punch and the pad. Then, at the time when thepushing-in of the punch into the die reaches the bottom dead point, theregions of the ridge line part and the vertical wall part are restrainedby the punch and the die. Consequently, when press working the ridgeline part including the shape-changing part, the material gathers to theregion of the shape-changing part and the region in the vicinity thereoffrom the peripheral regions, and it is likely that an excessive materialexcess is generated. This excessive material excess causes wrinkles inthe region of the shape-changing part and the region in the vicinitythereof.

In view of above, it can be said that the generation of wrinkles can bereduced by suppressing the excessive material excess in the region ofthe shape-changing part and the region in the vicinity thereof. In orderto do so, the region restrained by the punch and the pad from the earlyphase of the press working (hereinafter referred to as the “restrainedregion”) should be increased. The restrained region is not only theregion of the top panel part, but also the region of the shape-changingpart and a part of the region of the vertical wall part adjacent to theshape-changing part, where a wrinkle is likely to be generated. In thismanner, at the time of press working, there is no room for a materialexcess to be generated in the region of the shape-changing part and theregion in the vicinity thereof, and the generation of wrinkles isreduced.

The present invention has been completed on the basis of theabove-described findings. Hereinafter, a description will be given ofembodiments of the present invention with reference to the drawings.

First Embodiment

FIGS. 9 to 1 are perspective views showing an example of the structureof a producing apparatus used in producing a press formed product in afirst embodiment of the present invention. Each of the producingapparatuses 10 shown in FIGS. 9 to 11 is a press forming apparatus. Thepress forming apparatus 10 shown in FIG. 9 is an apparatus that producesthe first exemplary press formed product 7 having the hat-shapedcross-section shown in the above-described FIG. 2. The press formingapparatus 10 shown in FIG. 10 is an apparatus that produces the secondexemplary press formed product 7 having the hat-shaped cross-sectionshown in the above-described FIG. 3. The press forming apparatus 10shown in FIG. 11 is an apparatus that produces the sixth exemplary pressformed product 7 having the hat-shaped cross-section shown in theabove-described FIG. 7.

FIGS. 12A to 12C are diagrams showing the statuses of the press workingstep by the producing apparatus of the first embodiment. In thesefigures, FIG. 12A shows the state before starting the forming. FIG. 12Bshows the state of the early phase of the forming. FIG. 12C shows thestate at the completion of the forming. These figures show the crosssections of the region(s) of the shape-changing part(s).

As shown in FIGS. 9 to 12C, the press forming apparatus 10 includes apunch 11 as a lower die, and includes a die 12 and a pad 13 as an upperdie. That is, the punch 11 makes a pair with the die 12 and the pad 13.Note that FIGS. 9 to 11 show the die 12 by a dashed line, so as tofacilitate the understanding of the structure.

The punch 11 includes a top surface 11 a (a front end surface), a pairof side surfaces 11 b, and punch shoulder parts 11 c provided betweenthe top surface 11 a and the side surfaces 11 b. The punch shoulderparts 11 c are regions connecting the top surface 11 a to the sidesurfaces 11 b. The shape of the top surface 11 a is the shapecorresponding to the region of the top panel part 7 a of the pressformed product 7. The shapes of the side surfaces 11 b are the shapescorresponding to the region of the vertical wall parts 7 c of the pressformed product 7. The shape of the punch shoulder part 11 c is the shapecorresponding to the region of the ridge line part 7 b (including theshape-changing part 9) of the press formed product 7. The punch shoulderpart 11 c is provided with a shape-changing part in a part of thelongitudinal direction of the punch 11. The shape of the shape-changingpart of the punch shoulder part 11 c is the shape corresponding to theregion of the shape-changing part 9 that exists in a part of the ridgeline part 7 b of the press formed product 7. In other words, the punchshoulder part 11 c includes a portion where the cross-section shapechanges in the longitudinal direction of the punch 11, or where theextending direction of the punch shoulder part 11 c is bent.

The pad 13 is arranged opposed to the top surface 11 a of the punch 11.The pad 13 includes a bottom surface 13 a, a pair of inner side surfaces13 b, and corner parts 13 c. The corner parts 13 c are portions thatconnect the bottom surface 13 a to the inner side surfaces 13 b. Theshape of the bottom surface 13 a is the shape corresponding to theregion of the top panel part 7 a of the press formed product 7. That is,the bottom surface 13 a opposes the top surface 11 a of the punch 11,and its shape is the shape conforming to the top surface 11 a of thepunch 11. The shape of the corner part 13 c is the shape correspondingto the region of the ridge line part 7 b (including the shape-changingpart 9) of the press formed product 7. That is, the corner part 13 copposes the punch shoulder part 11 c, and its shape is the shapeconforming to the punch shoulder part 11 c. As used herein, “the shapeconforming” means the shape where the convexity and concavity of theopposing portions with the material metal sheet sandwiched therebetweenis inverted. The inner side surface 13 b opposes a part of the sidesurface 11 b of the punch 11. Specifically, the shape of the inner sidesurface 13 b is the shape corresponding to the region having apredetermined height h from the border between the vertical wall part 7c and the ridge line part 7 b of the region of the vertical wall part 7c of the press formed product 7 (see FIG. 12C).

The pad 13 is supported by an upper die holder integrally operated witha slide via a pressure member. The pressure member is a hydrauliccylinder, a gas cylinder, a spring, a rubber, etc., and applies adownward force (a force directed to the punch 11) to the pad 13. As longas the pad 13 is supported via the pressure member, the pad 13 may besupported by a slide that operates integrally with the upper die holder.

In the first embodiment, the die 12 is divided into two, and each of thedies 12 is arranged on each of the outer sides of the pad 13. Each ofthe dies 12 includes an inner side surface 12 a. The shape of the innerside surface 12 a is the shape corresponding to the region beyond thepredetermined height h of the region of the vertical wall part 7 c ofthe press formed product 7. That is, the inner side surface 12 a opposesa region of the regions of the side surface 11 b of the punch 11 exceptfor the regions opposing the inner side surface 13 b of the pad 13. Eachof the dies 12 is connected to the upper die holder or slide via a diemoving mechanism such as a cam. After the pushing-in of the punch 11into the pad 13 reaches the bottom dead point, and the pushing-in of themetal sheet 14 into the pad 13 by the punch 11 is completed, the diemoving mechanism moves each of the dies 12 obliquely downward to arespective one of the side surfaces 11 b of the punch 11 (see theoutlined arrows in FIG. 12B).

The producing method of the press formed product 7 using such pressforming apparatus 10 according to the first embodiment includes apreparation step and a press working step as follows.

[Preparation Step]

As shown in FIGS. 9 to 12A, a metal sheet 14 is prepared as a startingmaterial. For example, a high-strength steel sheet having a tensilestrength of 440 MPa or more can be used as the metal sheet 14. The metalsheet 14 may be a 590 MPa-class high-strength steel sheet, a 980MPa-class high-strength steel sheet, or even a 1180 MPa-classhigh-strength steel sheet. Additionally, a stainless steel sheet, analuminum sheet, a copper sheet, etc. may also be used as the metal sheet14.

[Press Working Step]

As shown in FIGS. 9 to 12C, in the press working step, press working bybending forming is performed on the metal sheet 14 by using the pressforming apparatus 10, so as to produce the press formed product 7.Hereinafter, a specific description will be given of the statuses in thepress working step.

As shown in FIG. 12A, after the metal sheet 14 is placed on the punch11, the slide is descended. In this manner, the pad 13 and the dies 12descend. Then, the pushing-in of the metal sheet 14 into the pad 13 bythe punch 11 is started, and the bending forming of the metal sheet 14is begun.

When the descending of the slide is further continued, the pushing-in bythe punch 11 with respect to the pad 13 reaches the bottom dead point,and the working by the punch 11 and the pad 13 is completed. In thismanner, as shown in FIG. 12B, the entire region of the top panel part 7a is formed. Also, the entire regions of the ridge line parts 7 bincluding the shape-changing parts 9 are formed. Further, the regionhaving the predetermined height h is formed in the region of each of thevertical wall parts 7 c.

After the working by the punch 11 and the pad 13 is completed asdescribed above, subsequently, the descending of the slide is continuedto the bottom dead point with the metal sheet 14 sandwiched andrestrained between the punch 11 and the pad 13. Then, each of the dies12 is moved, by the die moving mechanism, obliquely downward toward arespective one of the side surfaces 11 b of the punch 11, such that thedies 12 are become close to each other (see the outlined arrows in FIG.12B). In this manner, the working by the punch 11 and the dies 12 isbegun, and the region beyond the predetermined height h is formed in theregion of each of the vertical wall parts 7 c (see FIG. 12C). That is,the remaining regions are formed by the punch 11 and the dies 12.Further, in this embodiment, the flange parts 7 d, which are connectedto the vertical wall parts 7 c, are also formed by the punch 11 and thedies 12. In this way, the press formed product 7 is obtained.

According to the press working step using the press forming apparatus 10as described above, the region of the shape-changing part 9 and a partof the region of the vertical wall part 7 c adjacent to theshape-changing part 9, where a wrinkle is likely to be generated, arerestrained by the punch 11 and the pad 13 from the early phase of theworking. Therefore, at the time of press working, there is no room for amaterial excess to be generated in the region of the shape-changing part9 and the region in the vicinity thereof. As a result, even when ahigh-strength steel sheet is used as the starting material, it ispossible to suppress the generation of wrinkles in the region of theshape-changing part 9 and the region in the vicinity thereof.Accordingly, in the case where the press formed product having arelatively complex shape is produced, it is possible to expedite theincreasing of the strength, and it is also possible to increase thedegree of freedom in designing.

As described above, the region of each of the vertical wall parts 7 cadjacent to the shape-changing part 9 is formed such that, first, theregion having the predetermined height h is formed by the pad 13 and thepunch 11, and thereafter, the remaining regions are formed by the dies12 and the punch 11. It is preferable that the predetermined height h is2 mm or more from the border between the vertical wall part 7 c and theridge line part 7 b, and is equal to or less than a half of the totalheight H (H/2) of the vertical wall part 7 c. The reason is as follows.In a case where the predetermined height h is less than 2 mm from theborder between the vertical wall part 7 c and the ridge line part 7 b,at the time of the subsequent working by the dies 12 and the punch 11,the working regions of the vertical wall parts 7 c become broad, and awrinkle is likely to be generated. On the other hand, when thepredetermined height h is more than a half of the total height H of thevertical wall part 7 c, at the time of the aforementioned working by thepad 13 and the punch 11, the working regions of the vertical wall parts7 c become broad, and also in this case, a wrinkle is likely to begenerated. However, the lower limit for the predetermined height h ispreferably 3 mm, and more preferably 5 mm. On the other hand, the upperlimit for the predetermined height h is preferably 40 mm, and morepreferably, 20 mm.

The press formed product 7 to be produced in the first embodiment is notlimited to the first, second and sixth exemplary press formed product 7having the hat-shaped cross-section shown in FIGS. 2, 3 and 7. That is,the press formed product 7 may be the third to fifth and seventhexemplary press formed products 7 shown in the above-described FIGS. 4to 6 and FIG. 8, as long as a part of the ridge line part 7 b includes ashape-changing part 9. In short, the shape-changing part 9 of the ridgeline part 7 b of the press formed product 7 is at least one of (a) to(f) as follows:

(a) the height of the ridge line part is changed;

(b) the arc length in the cross section of the ridge line part ischanged;

(c) the ridge line part is twisted;

(d) the ridge line part is bent in the width direction;

(e) the ridge line part protrudes or is hollowed in the width direction;and

(f) the radius of curvature in the cross section of the ridge line partis changed.

Note that, in the first exemplary press formed product 7 shown in theabove-described FIG. 2, cracks are likely to be generated in the regionsof the shape-changing parts 9 and in the region in the vicinity thereof.According to the first embodiment, it is also possible to suppress suchcracks.

Additionally, the pair of ridge line parts of the press formed productdoes not have to be parallel. For example, a pair of ridge line partsmay be crossed at their ends. Additionally, the cross-section shape ofthe press formed product may be a channel shape without the flangeparts.

Second Embodiment

FIGS. 13A to 13C are diagrams showing the statuses of the press workingstep by a producing apparatus according to a second embodiment. Amongthese figures, FIG. 13A shows the state before starting the forming.FIG. 13B shows the state of the early phase of the forming. FIG. 13Cshows the state at the completion of the forming. These figures show thecross sections of the region(s) of the shape-changing part(s), as inFIGS. 12A to 12C.

A second embodiment is based on the first embodiment, and modifies apart of the first embodiment. In the second embodiment, the height ofthe inner side surface 13 b of the pad 13 is larger compared to theabove-described first embodiment. Each of the dies 12 is arrangedadjacent to the either side of the pad 13, and is fixed directly to theupper die holder. That is, no special die moving mechanism is provided.Further, in the case of the second embodiment, the die 12 may beintegral without being divided into two.

As in the above-described first embodiment, in the press working step ofthe second embodiment, first, the working by the punch 11 and the pad 13is completed with the descending of the upper die holder (see FIG. 13B).Then, the descending of the upper die holder is subsequently continued.Consequently, each of the dies 12 descends directly, and reaches to thebottom dead point (see the outlined arrows in FIG. 13B). In this manner,in each of the regions of the vertical wall parts 7 c, the region beyondthe predetermined height h is formed by the punch 11 and the die 12 (seeFIG. 13C). In this way, the press formed product 7 is obtained.

According to the press working step of the second embodiment, it is alsopossible to reduce the generation of wrinkles in the region of theshape-changing part 9 and the region in the vicinity thereof, as in theabove-described first embodiment. Additionally, in the secondembodiment, it is unnecessary to provide a special die moving mechanism.Therefore, the apparatus configuration is simple.

Third Embodiment

FIGS. 14A to 14C are diagrams showing the statuses of the press workingstep by a producing apparatus according to a third embodiment. Amongthese figures, FIG. 14A shows the state before starting the forming.FIG. 14B shows the state of the early phase of the forming. FIG. 14Cshows the state at the completion of the forming. These figures show thecross sections of the region(s) of the shape-changing part(s), as inFIGS. 12A to 12C, and FIGS. 13A to 13C.

A third embodiment is based on the second embodiment, and modifies apart of the second embodiment. In the above-described second embodiment,the substantial working regions of the vertical wall parts 7 c by thedies 12 and the punch 11 are front end parts (lower bottom parts) of thevertical wall parts 7 c, and are significantly narrow. Therefore, theaddition of a restriking step for fine adjusting the shape may berequired. The third embodiment improves such a disadvantage (an increasein the steps) of the second embodiment.

Each of the dies 12 in the third embodiment is arranged on either outerside of the punch 11. Each of the dies 12 is connected to a lower dieholder or bolster via a die moving mechanism such as a cam. Each of thedies 12 may be connected to the upper die holder or slide via the diemoving mechanism such as a cam, as in the above-described firstembodiment. However, after the pushing-in of the punch 11 with respectto the pad 13 reaches the bottom dead point, and the pushing-in of themetal sheet 14 into the pad 13 by the punch 11 is completed, the diemoving mechanism in the third embodiment moves each of the dies 12horizontally toward a respective one of the side surfaces 11 b of thepunch 11 (see outlined arrows in FIG. 14B).

As in the above-described first embodiment, in the press working step ofthe third embodiment, first, the working by the punch 11 and the pad 13is completed with the descending of the upper die holder (see FIG. 14B).Then, the descending of the upper die holder is subsequently continuedto the bottom dead point. Consequently, each of the dies 12 is moved, bythe die moving mechanism, horizontally toward a respective one of theside surfaces 11 b of the punch 11, such that the dies 12 become closerto each other (see the outlines arrows in FIG. 14B). In this manner, inthe region of each of the vertical wall parts 7 c, the region beyond thepredetermined height h is formed (see FIG. 14C). In this way, the pressformed product 7 is obtained.

According to the press working step of the third embodiment, it is alsopossible to reduce the generation of wrinkles in the region of theshape-changing part 9 and the region in the vicinity thereof, as in theabove-described first and second embodiments. Additionally, in the thirdembodiment, it is possible to suppress the addition of the restrikingstep, which may occur in the second embodiment, because the substantialworking regions of the vertical wall parts 7 c by the dies 12 and thepunch 11 are broad.

Also, needless to say, the present invention is not limited to theabove-described embodiments, and various modifications can be madewithout departing from the spirit of the present invention. For example,though the press forming apparatus of the above-described embodimentshas the structure that includes the punch as the lower die, and includesthe die and the pad as the upper die, the press forming apparatus of theabove-described embodiments may have the structure in which thearrangement of the upper and lower dies is inverted.

Additionally, the restrained region of the top panel part by the punchand the pad may be at least a region adjacent to the shape-changing partof the region of the top panel part. The restrained region of the ridgeline part by the punch and the pad may be at least a region of theshape-changing part in the region of each of the ridge line parts. Therestrained region of the vertical wall part by the punch and the pad maybe at least a region adjacent to the shape-changing part and up to thepredetermined height h in each of the vertical wall parts. For example,in the region of the top panel part, those regions that are not adjacentto the shape-changing part(s) may be formed by the pad, or may be formedby a separate pad. In the regions of the ridge line parts, those regionsexcept for the shape-changing part(s) may be formed by the pad, or maybe formed by the dies. In the regions of the vertical wall parts, thoseregions that are not adjacent to the shape-changing part(s) may beformed by the pad, or may be formed by the dies.

Example

The degree of generation of wrinkles was studied in the case ofproducing the second exemplary press formed product 7 shown in theabove-described FIG. 3. As the Inventive Example of the presentinvention, an FEM analysis was conducted assuming the press workingaccording to the first embodiment shown in the above-described FIGS. 12Ato 12C. As a Comparative Example, an FEM analysis was conducted assumingthe pad bending forming shown in the above-described FIG. 1. In each ofthe analyses, a DP steel having a 590 MPa-class tensile strength and asheet thickness of 1.2 mm was used as the starting material. Then, ineach of the Inventive Example of the present invention and theComparative Example, the sheet thickness increase rate was calculatedfor the shape-changing part, where it was feared that a wrinkle might begenerated. The higher the sheet thickness increase rate, the larger thematerial excess becomes, and a wrinkle is likely to be generated.

As used herein, the sheet thickness increase rate A is expressed by thefollowing Formula (1).

A=(t1−t0)/t0×100[%]  (1)

In Formula (1), t0 represents the sheet thickness before the pressworking, and t1 represents the sheet thickness after the press working.

FIG. 15 is a graph showing the study result of the example. As shown inFIG. 15, in the Comparative Example, the sheet thickness increase ratein the shape-changing part was more than 15%, and the generation ofwrinkles was expected. In contrast, in the Inventive Example of thepresent invention, the sheet thickness increase rate in theshape-changing part was suppressed to about 4%, which was the level atwhich a wrinkle was not generated.

REFERENCE SIGNS LIST

-   7: press formed product-   7 a: top panel part-   7 b: ridge line part-   7 c: vertical wall part-   7 d: flange part-   8: step height part-   9: shape-changing part-   10: press forming apparatus-   11: punch-   11 a: front end surface-   11 b: side surface-   11 c: punch shoulder part-   13: pad-   13 a: bottom surface-   13 b: inner side surface-   13 c: corner part-   12: die-   12 a: inner side surface-   14: starting material (metal sheet)

1. A method for producing a press formed product from a startingmaterial, the press formed product comprising a top panel part, verticalwall parts respectively extending from both ends of the top panel part,and ridge line parts between the vertical wall parts and the top panelpart, and including a shape-changing part in a part of the ridge lineparts, the producing method comprising: a preparation step of preparinga metal sheet as the starting material; and a press working step ofperforming press working on the starting material by using a punch, anda pad and a die that are paired with the punch, and the press workingstep including: a first step of forming, by the punch and the pad, atleast a region adjacent to the shape-changing part of a region of thetop panel part, at least a region of the shape-changing part of a regionof each of the ridge line parts, and at least a region adjacent to theshape-changing part of a region of each of the vertical wall parts andup to a predetermined height from a border between the vertical wallpart and the ridge line part; and a second step of forming, subsequentto the first step, by the punch and the die, the remaining regions, withthe starting material sandwiched between the punch and the pad.
 2. Theproducing method of the press formed product according to claim 1,wherein the predetermined height is 2 mm or more from the border betweenthe vertical wall part and the ridge line part, and is equal to or lessthan a half of the total height of the vertical wall part.
 3. Theproducing method of the press formed product according to claim 1,wherein, in the first step, the entire region of the top panel part, theentire region of each of the ridge line parts, and a region up to thepredetermined height from the border between the vertical wall part andthe ridge line part of a region of each of the vertical wall parts areformed by the punch and the pad, and in the second step, a region beyondthe predetermined height of the region of each of the vertical wallparts is formed by the punch and the die.
 4. The producing method of thepress formed product according to claim 1, wherein the shape-changingpart of the ridge line parts is at least one of (a) to (f) as follows:(a) the height of the ridge line part is changed; (b) the arc length ina cross section of the ridge line part is changed; (c) the ridge linepart is twisted; (d) the ridge line part is bent in a width direction;(e) the ridge line part protrudes or is hollowed in the width direction;and (f) the radius of curvature in the cross section of the ridge linepart is changed.
 5. An apparatus for producing a press formed productfrom a starting material, the press formed product comprising a toppanel part, vertical wall parts respectively extending from both ends ofthe top panel part, and ridge line parts between the vertical wall partsand the top panel part, and including a shape-changing part in a part ofthe ridge line parts, the producing apparatus comprising a punch, and apad and a die that are paired with the punch, the punch including afront end surface corresponding to a region of the top panel part, sidesurfaces corresponding to regions of the respective vertical wall parts,and punch shoulder parts corresponding to regions of the respectiveridge line parts, and the pad including a bottom surface correspondingto at least a region adjacent to the shape-changing part of a region ofthe top panel part, a corner part corresponding to at least a region ofthe shape-changing part of a region of each of the ridge line parts, andan inner side surface corresponding to at least a region adjacent to theshape-changing part of a region of each of the vertical wall parts andup to a predetermined height from a border between the vertical wallpart and the ridge line part.
 6. A producing apparatus of a press formedproduct, comprising: a punch including a front end surface, sidesurfaces, and punch shoulder parts provided between the front endsurface and the side surfaces, the punch shoulder parts being providedwith a shape-changing part in a part of a longitudinal direction of thepunch; a pad including a bottom surface opposing the front end surfaceof the punch, inner side surfaces opposing a part of the side surfacesof the punch, and corner parts provided between the bottom surface andthe inner side surfaces and opposing the punch shoulder parts; and a dieincluding inner side surfaces opposing regions of the side surfaces ofthe punch, except for regions opposing the inner side surfaces of thepad of the regions of the side surfaces of the punch.
 7. The producingapparatus of the press formed product according to claim 5, wherein thepredetermined height is 2 mm or more from the border between thevertical wall part and the ridge line part, and is equal to or less thana half of the total height of the vertical wall part.
 8. The producingapparatus of the press formed product according to claim 5, wherein thebottom surface of the pad corresponds to the entire region of the toppanel part, each of the corner parts of the pad corresponds to theentire region of each of the ridge line parts, and each of the innerside surfaces of the pad corresponds to a region up to the predeterminedheight from the border between the vertical wall part and the ridge linepart of a region of each of the vertical wall parts, and wherein the dieincludes inner side surfaces corresponding to regions beyond thepredetermined height of regions of the respective vertical wall parts.9. The producing apparatus of the press formed product according toclaim 8, wherein the die is divided to correspond to each of thevertical wall parts, and wherein the producing apparatus comprises a diemoving mechanism that moves each of the dies toward a respective one ofthe side surfaces of the punch after completion of pushing-in of thestarting material into the pad by the punch.